The invention relates to a method of centerless grinding and to abrasives tools used in centerless grinding and other cylindrical surface grinding processes designed to permit the option of small or large volume stock removal in the production of many categories of diverse parts.
Centerless grinding is a process for rapid, accurate, surface grinding of difficult to hold parts. The part to be ground, i.e., the workpiece, is rigidly supported directly under the cut without application of end pressure to the part, allowing heavier or deeper cuts to be made and making it possible to grind long, brittle or easily distorted parts. Among the parts manufactured by centerless grinding are straight and tapered bearings, rollers, bars, needle rollers, bushings, bolts, fasteners, pistons, piston rings, gun barrels, rods, shafts, shells, tappets, pen parts, hypodermic needles, forgings and numerous other items made of various metal, plastic, ceramics and composite materials.
Centerless grinding differs from other types of grinding in that the workpiece is not suspended between centers or by other fixtures connected to an end or a surface of the workpiece. Instead, the workpiece rests on a blade or support, and a regulating wheel, most often made of a rubber material, contacts the workpiece urging it against the support and against a grinding wheel. In most common systems, the grinding wheel rotation also rotates the workpiece, the cutting pressure developed by the grinding wheel forces the workpiece against the regulating wheel and the support, and the regulating wheel governs the speed of rotation of the workpiece. Thus, the grinding wheel and the workpiece can be rotated at different revolutions per minute (rpms). For example, a grinding wheel speed of 7,500 surface feet per minute (sfpm) may be used with a regulating wheel speed (and matching workpiece speed) of 36 to 900 spfm. Continuous or semi-automated grinding processes are possible with centerless grinding as parts can be continuously fed into the system, so long as the grinding wheel remains within specifications.
Thus, there exists a continual demand in grinding operations for improved grinding wheels for centerless grinding, wherein the wheels have consistent profiles throughout the body of the wheel as it grinds, the wheels are resistant to excessive wear and the wheels are effective in removing stock from workpieces leaving a smooth, uniform consistent part size, shape and finish.
In the past, grinding wheels for centerless grinding typically were improved by increasing their hardness grade by means of reducing the porosity of the wheel, increasing the abrasive grain and bond content and/or increasing the density of the abrasive composite making up the wheel. In general, these steps increased the grinding efficiency of any given process, i.e., the G-ratio (material removal rate/wheel wear rate or MRR/WWR), up to the point where the forces of grinding with these harder wheels began to interfere with part quality or exceeded the power capacity of the machine or, particularly in the case of organic bonded wheels, increased the wheel wear rate through excessive thermal degradation of the bond and premature release of unused abrasive grain from the abrasive composite.
It has now been discovered that certain abrasive tools having lower hardness grades exhibit improved grinding efficiency in centerless grinding processes and other grinding processes as a result of the material properties and microstructure of the abrasive composite, in particular, the means by which the abrasive grain is anchored within the composite. These abrasive tools perform in a significantly more efficient manner than the best prior art abrasive tools, especially when considered on the basis of volume of abrasive grain required to remove equivalent amounts of stock from a workpiece. The tools have utility in foundry grinding and snagging, and in track, bar and needle grinding, where higher density abrasive tools have been used, as well as in centerless grinding.